1. Field of the Invention
The present invention relates to an ink jet recording apparatus used in a printer or in a facsimile machine for example, and to an ink jet recording method.
2. Description of the Related Art
Conventionally, there is known an ink jet recording apparatus that records characters and images on a recording medium using a ink jet head which ejects ink from a plurality of nozzles. In such an ink jet recording apparatus, the nozzles of the ink jet head are provided in a head holder so as to oppose the recording medium, and this head holder is mounted on a carriage to be scanned in a direction orthogonal to a conveying direction of the medium to be recorded.
A schematic exploded view of an example of a head chip of such an ink jet head is shown in FIG. 13 and a sectional view of main parts of the same is shown in FIG. 14. As shown in FIGS. 13 and 14, a plurality of chambers 102 are provided in parallel with each other in a piezoelectric ceramic plate 101, and each chamber 102 is separated by side walls 103. An end portion in a longitudinal direction of each chamber 102 is extended to an end surface of the piezoelectric ceramic plate 101 and the other end portion is not extended to the other end surface, making the chamber 102 become gradually shallower. In addition, electrodes 105 for applying a driving electric field are formed on surfaces on opening-side of both the side walls 103 in each chamber 102 along its longitudinal direction.
A cover plate 107 is bonded to the piezoelectric ceramic plate 101 on the opening side of the chambers 102 by using adhesive 109. The cover plate 107 includes a common ink chamber 111 to be a recessed portion communicating with the other end portion of each chamber 102 where it is shallower, and an ink supply port 112 that is bored from the bottom portion of this common ink chamber 111 in a direction opposite to the chamber 102.
In addition, a nozzle plate 115 is bonded to an end surface of a bonded body of the piezoelectric ceramic plate 101 and the cover plate 107 in which the chambers 102 are opened, and nozzle openings 117 are formed in the nozzle plate 115 at positions opposing the respective chambers 102.
Note that, a wiring substrate 120 is fixed to the surface of the piezoelectric ceramic plate 101 which is on the side opposite from the nozzle plate 115 and on the side opposite from the cover plate 107. Wiring 122 connected to each electrode 105 by bonding wires 121 and the like is formed on the wiring substrate 120, and a driving voltage can be applied to the electrodes 105 via this wiring 122.
In a head chip configured in this way, when each chamber 102 is filled with ink from the ink supply port 112 and a predetermined driving electric field is caused to act on the side walls 103 on both sides of the predetermined chamber 102 via the electrode 105, the side walls 103 are deformed to change the volume of the predetermined chamber 102, whereby the ink in the chamber 102 is ejected from the nozzle opening 117.
For example, as shown in FIG. 15, when ink is to be ejected from the nozzle opening 117 corresponding to a chamber 102a, a positive driving voltage is applied to electrodes 105a and 105b within the chamber 102a, and electrodes 105c and 105d which face the electrodes 105a and 105b, respectively, are grounded. This causes a driving electric field to act on side walls 103a and 103b in a direction towards the chamber 102a, and if this is orthogonal to the polarization direction of a piezoelectric ceramic plate 101, the side walls 103a and 103b deform towards the chamber 102a due to a piezoelectric thickness shear effect to reduce the volume of the chamber 102a while increasing the pressure. Thus, ink is ejected from the nozzle opening 117.
In such a head chip, although the time required from when vibration of the side walls caused by ink ejection stops until when the ink pressure in the chamber becomes zero to be ready for next ink ejection depends upon the length of the chamber, the shape of the nozzle opening, and the like, since the chamber is low in sealing property, the sound pressure is repeatedly reflected within the chamber, thus requiring a considerable amount of time for completely attenuating it. Therefore, a problem occurs in that it is difficult to increase the speed of continuous ejection, that is, to increase the printing speed.
Since the time required until the sound pressure attenuates largely varies depending upon the shape of the nozzle opening, in particular, a problem occurs in that it is difficult to control the amount of ejection according to the shape of the nozzle opening.
The chamber is composed of a boundary portion communicating with the common ink chamber, and a pump portion extending from the nozzle opening to the boundary portion, which is driven to eject ink, and the contraction time during which the chamber pressure attenuates depends upon the length of the pump portion, i.e., upon the distance from the nozzle opening to the boundary portion. However, the problem is that, when the pump length is shortened in order to reduce the contraction time, ink ejection characteristics are deteriorated, resulting in unnormal printing operation.
A driving electric field generated in the side walls on both sides of the chamber by one-time ejection consists of a preliminary driving electric field which causes the chamber volume to temporarily increase, and an ejection driving electric field which causes the chamber volume to temporarily decrease subsequently to the preliminary driving electric field. The driving time ratio of the preliminary driving electric field to the ejection driving electric field is AP to 2Nxc3x97AP (N denotes a natural number, and AP denotes a periodic time that is determined by the pump length and the pressure propagating speed within ink, i.e., a time required from a positive pressure peak to a negative pressure peak). That is, as the preliminary ejection driving generates a negative pressure in the chamber, and the ejection driving electric field generates a positive pressure in the chamber; after the positive pressure has been generated by the ejection driving electric field, the chamber volume is returned to the original volume, thereby causing a negative pressure to be generated in the chamber, and a positive pressure peak generated after 2Nxc3x97AP after generation of an ejection driving electric field is cancelled by this negative pressure, thus preventing ink leakage or ejection failure.
Therefore, if the driving time ratio of the preliminary driving electric field to the ejection driving electric field is AP to (2Nxe2x88x921)xc3x97AP, that is, if the ejection driving electric field is an odd-numbered multiple, a negative pressure peak generated at a period of 2AP after a time AP elapses since an application of ejection driving electric field coincides with the timing of generation of a negative pressure caused by the chamber volume returning to the original volume, thus extraordinarily increasing the negative pressure to thereby cause contamination of air bubbles into the chamber or degradation in the ejection performance. Therefore, the driving time ratio of the preliminary driving electric field to the ejection driving electric field is set to AP to 2Nxc3x97AP, so that a positive pressure peak generated at a period of 2AP after a time 2AP elapses after an application of ejection driving electric field is cancelled by a negative pressure generated when the chamber volume returns to the original volume. The time involved in ejection using both the preliminary driving electric field and the ejection driving electric field becomes at least 3AP, thus requiring a long ejection time and further requiring the time for contraction of the pressure in the chamber for next ejection. Therefore, a problem occurs in that it is difficult to increase the speed of continuous ejection, in particular.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide an ink jet recording apparatus and recording method in which the contraction time during which the pressure in a chamber attenuates is reduced and the driving time is reduced to thereby increase the printing speed without deterioration of ink ejection characteristics.
To solve the above-described problems, according to a first aspect of the present invention, there is provided an ink jet recording apparatus comprising: a head chip having chambers which are defined in a substrate and whose end portions in the longitudinal direction thereof communicate with nozzle openings, and electrodes provided on side walls of the chambers; and driving means for applying driving voltages to the electrodes of the head chip to generate driving electric fields in the side walls to change the volumes of the chambers, thereby causing ink filled therein to be ejected from the nozzle openings, the apparatus being characterized in that: an ink chamber plate for defining a common ink chamber communicating with the chambers is bonded on the substrate; the common ink chamber is provided with a partitioning portion for partitioning the chambers and the common ink chamber, the partitioning portion being provided with a plurality of communicating holes for defining a pump length according to the distance from the nozzle openings, along the longitudinal direction of the chambers at an interval equivalent to the pump length; and the driving means performs driving so as to make substantially equal the driving time of a preliminary driving electric field which causes the volumes of the chambers to temporarily increase and the driving time of an ejection driving electric field which causes the volumes of the chambers to temporarily decrease subsequently to the preliminary driving electric field to cause the ink to be ejected, as the driving electric fields to be generated on the side walls.
A second aspect of the present invention relates to the ink jet recording apparatus according to the first aspect of the invention, characterized in that the partitioning portion is formed of a different member.
A third aspect of the present invention relates to the ink jet recording apparatus according to the first or second aspect of the invention, characterized in that the substrate is formed of a piezoelectric ceramic plate, and grooves are formed in the piezoelectric ceramic plate to define the chambers, the chambers communicating with the common ink chamber at openings in end portions in the longitudinal direction of the chambers which are opposite from the substrate.
A fourth aspect of the present invention relates to the ink jet recording apparatus according to the first or second aspect of the invention, characterized in that the side walls made of piezoelectric ceramic are arranged at a predetermined interval on the substrate, and the chambers are defined within the side walls and the common ink chamber is defined in the substrate, the chambers and the common ink chamber being communicated with each other at one end in the longitudinal direction of the chambers.
According to a fifth aspect of the present invention, there is provided an ink jet recording method which comprises applying voltages to electrodes of a head chip that comprises: a substrate in which chambers whose end portions in the longitudinal direction thereof communicate with nozzle openings are defined and the electrodes are provided on side walls of the chambers; and an ink chamber plate bonded on the substrate to define a common ink chamber communicating with the chambers, to thereby change the volumes of the chambers to cause ink filled therein to be ejected from the nozzle openings, the method being characterized in that: the common ink chamber is provided with a plurality of communicating holes for defining a pump length according to the distance from the nozzle openings, along the longitudinal direction of the chambers at an interval equivalent to the pump length; and, as the driving electric fields, a preliminary driving electric field which causes the volumes of the chambers to temporarily increase and an ejection driving electric field which causes the volumes of the chambers to temporarily decrease subsequently to the preliminary driving electric field are generated in the side walls for substantially equal driving time.
According to the present invention as described above, the provision of communication holes for defining the pump length of the chamber enables a reduction of time during which the pressure in the chamber attenuates, and enables a reduction of time during which an ejection driving electric field is generated to reduce the driving time involved in the ejection, without deterioration of ink supply characteristics and ink ejection characteristics, thereby achieving high-speed printing with continuous ink ejection at a high speed.